This invention relates to self-propelled, material-handling vehicles, and more particularly relates to such vehicles which employ an extendible boom.
Over the past decade, experience has revealed the need for a material-handling vehicle which can be used in the construction, manufacturing and warehousing industries. Such a vehicle must have the reach capability of a conventional overhead crane and the maneuverability and compactness generally attributed to a conventional fork lift truck. In order to be useful, such a vehicle must have a boom capable of raising a heavy load to the top of a multi-story building and must have a low profile and short wheel base that enable operation within a warehouse or stock-piling yard.
In the past, vehicles capable of lifting heavy loads to a substantial height have not been capable of also performing as a conventional fork lift truck. One such prior art machine comprises a vehicle body mounted on wheelbearing front and rear axles. A cab for a driver is centrally located over the front axle, and a boom is rotatably mounted on a pivot axis located substantially over the rear axle and above the top of the cab so that the boom extends forward over the cab. A fork is attached to a support member that extends in front of the cab and is connected to the free end of the boom. An engine which powers the vehicle is located over the rear axle, and the major portion of the engine is located above the rear wheels.
While such a vehicle is capable of lifting loads to a substantial height, the vehicle is relatively useless in an environment requiring small size and unimpaired visibility by the driver, such as a warehouse. The mounting of the boom above the cab creates a vehicle which is too high to go through many warehouse archways, and the support member extending in front of the cab impairs the vision of the driver. Moreover, the vehicle has a relatively high center of gravity which increases the moment arm created around the front axle during deceleration of the vehicle, such as during braking. When the vehicle is carrying a heavy load on the fork, this moment arm tends to pitch the vehicle forward and to lift the rear wheels off the ground during braking. The problem is accentuated when the boom is extended to any substantial extent. As a result of this deficiency, the vehicle tends to be unstable when it is maneuvered with a heavy load.
In addition to the foregoing deficiencies, the vehicle provides no means for simultaneously carrying a load on the fork and a portion of the vehicle body. This deficiency limits the physical size of the load which can be safely carried by the vehicle. The vehicle also is not capable of maintaining a load at the same elevation above the ground level as the boom is extended and retracted. This deficiency limits the ability of the driver to conveniently maneuver a load. In addition, the position of the boom over the cab of the vehicle prevents the boom from extending into an excavation so that a load may be maneuvered below the grade level.